Plural channel secrecy system



. designate Patented Apr. 17.1928.

` UNITED. STATES PA'raNro RAYMOND t mutisme, oF MILLBURN, NEW JERSEY, lassioivon To Wiesmann mc- Yorin incassi F F lCEf TRIC jCOMPANY, INCOBPORATED,.0F NEW kYQRX, N. Y., A yGQRPOBTIDN OFNEW uumlr. siicnncYsYsrmL' Application inea December so, 192s. serieu in. massa.

This invention relates to-multiplex wave transmission systems.

An object of the inventionis to provide for the transmission of several messages over the same line or circuit in such a manner as to make itverydiliicult for those not in possession of aspecial and peculiar receivingfapparatus to separate and distinguish the various messages from each other. j c Consequently, an important feature of the invention is the element of secrecy which is introduced. i v

A further object of the invention is to so scramble or intermingle the kwave frequency components characteristic of several sets of waves representing messages, that-,their unsci-ambling or separation can be effected only by specially designed and constructedap' paratus.

'A further object is to 4provide for the transmission of a maximum number of messages through a channelcomprising a minimum range of frequencies or wave lengths. A still further object is to rovide for the transmission of several tele one conversations ovei' a line without utilizing the ran es of frequencies below about 300 to 500 cyc es, thereby leaving available this range for Morse'transmission or other purposes; and furthermore, leaving available a considerable range of frequencies above the general region of 6500 cycles to 10,000 cycles for car#l rier or other'transmission. n

A still further object is to provide for du- `plix communication 2in 'which thev foregoing objects are fulfilled for' transmissionjin each direction.

In a preferred embodiment o f the inven-kv tion there is provided at a transmitting terminal ap Aaratus which may generally d) as scrambling or intermingling apparatus, and at a' receiving terminal sepa-J rating or unsc'rambling apparatus. v Several speech bands at the transmitting terminal are each subdividedinto several portions by `means of`band-passjiilte'rs or equivalent means.v These several'SDeeCh sub-bands are then translated in fre uency and so superimfposed uponpone anot er ina common circuit that their'frequency'ranges lie one above fthe other. The` `various, band subdivisions when su erimposed are'so arranged that'no twosubdivisions or subfbands constituting a portion of the same message lie adjacent each other. At the .receiving terminal' the combined to reproduce -`frequency speech bands. v

cles

various band lsubdivisions are li'rstseparated from each otherby means 'of band-pass til-4 teis or equivalent' elements fand thereafter translatedin frequency to ,reproduce the original band subdivisions whichare'tl'ien rethe `vfifii'iginal voice The invention is "more .particnlarl described hereinafter; in connectionlwitht e 30,-] coinpanying drawings wherein Fig." '1 is adiagram indicating how the frequenciescorrespondin upon the 'other to constitute a scrambled for scrambling or.' confusing the messages,

' and a receiving terminal for unscrambling or to several conversations f desi nated @Speech A, fSpeech B and S ec i (l"` are recombined and superimpose one .combination of the three messages; Fig. 2is -a .circuit diagram ofl a transmitting terminal use of in several portionsof the circuit-of BF; Fig. 4 is a detailed circuit s owing-a. preferred type of modulating and detecting diagraml indicatingv a referred manner'l'o'f two-way systems; Fig. 6 illustrates the apiig. 2f au. such band filters `,bein marked .or' demodulatingfcircuit which is made use 'i of in the circuit of Fig. 2; Fig. 5 isa circuitv plic'at'ion of the circuit of 'Fig. 2 to radio transmission; Fig. 6'i illustrates the application of the system. of Fig. 2 to du lex or twof 'way radio communication; and ig. 7 indicates a method of combining the arrangements of Fig. 2 with additional modulating,V

apparatus for carrier wave transmission.

In the left hand portion offFig. 1 are diagrams indicating how several bands of speech frequencies are to be divided. Taking the `band marked .Speech A for example, it wi1l be seen that the band from zero to-500 cycles is Adiscarded and the'ban'd from y500 cycles to2500 cycles is divided into three' sub-bands, A, extending from 500 eycls' to 900 cycles, A2 extending fi'om`900 cycles to 1600 cycles, and/Ai3 extending from 1600 cycles to 2500 cycles. above 2500 cycles are also discarded.

' 1 It is somewhat unusual in telephonewbrk to discard speech frequencies below 500'c cles, but, as has been shown by the recent y Frequencies published data of certain investigators, the` range of frequencies extending from 200 cyto 500 cycleais of less impotancethan uitV l v,frequencies may be in frequency range as desired. It is precoming over a line or ferred, however, to divide each speech band into several sub-bands of different widths, inasmuch as this will increase the diiculties of anyone who should perchance attempt to construct apparatus designed to separate the various messages. By making the upper frequency hands of greater width the total energy and importance of the bands is made .more nearly equal than would be the case with hands of equal width.

Having- ,considered the separation of several conversations, 'as for example, Speech A, S eech B, and Speech C into several su -bands, consideration will now be iven to apparatus for reassemhlin these hands in a single circuit in the scram led or intermingled relation indicated in the right hand diagram of Fig. l.

In Fig. 2, Speech A is assumed to he in circuit l, Speech B over a line or circuit 2 and Speech C over a line or circuit 3. Appropriate hand lters a havin transmission ranges as indicated on the rawing, serve to separate the waves constituting Speech A, other hand lters a serve to separate the waves of Speech l5, and other hand lters c serve to separate the `Waves of Speech C.

Using the terminology indicated on the diagram of Fig. l, one sub-hand from each conversation is transmitted to 'the channel 4 which, as We may assume, will carry subhands which are designated as A1, B, and C3. Sub-bands B1, C, and A, are impressed together upon a modulating circuit 5 of the type# hereinafter described. The frequency of the carrier wave which is supplied to the modulatin@ circuit 5 from a source 6 is l0,000 cycllas. A hand iilter 7 in the output circuit of the modulator 5 passes a range of lrequencies o from l0,500 to 12,500. n a similar manner the sub-hands C A2 and B3 are applied to the input circuit of a modu-` lator 8 in the output circuit of which is a hand filter 9- Which passes waves of the range of frequencies of from 12,500 to 14,500, which result from` the application to 'the modulator 8 of a carrier wave the `frequency of which is 12,000 cycles. All the Waves transmitted through hand filters 7 and 9 'are impressed upon the input circuit- ;ofa modulator 10 to which; is' supplied a 'carrier fre.

quency of 8,800'cycies. In the vout-put circuit of the modulator 10 there vivill be present I among other frequencies, a band of frequencies of from 2,500 to,6,500 cycles. These ma be passed through a band filter 11 which passes frequencies Within this range and excludes other frequencies. However,

`the band filter 11 may beomitted if desired.

Waves of a range of frequencies of from 500 to 6,500 are consequently' impressed upon the `terminals l12 of a transmission line L.

These Waves are transmitted over the line L which may beany suitable transmission circult of any nature lwhatsoever, to the line terminals 13.l It will be observed'that `a total of three conversations is traversing the line L and that all three of them occupy a frequenc range ofv only 6,000 cycles, leav-` ing avai ble for other purposes the range belowV 500 cycles and the range above 6,500 c cles. Furthermore, the various frequencies of the Vthree conversations are so intermingled with each other that it would be a Y matter of considerable diiculty to one, unacquainted with' the terminal apparatus, to separate out the waves representlng any particular conversation., If it is ydesired to insure that some noise current-s will alwa s be present upon the line, there may be su stituted for one of the conversations vA, B or C, ano1se producing `device such as a phonograph record running forward or backward, or othery similarv arrangement.

Such an arrangement 'is illustrated by .the .v

noise producing device or phonograph 50 impresjng speech or noise upon the microphone 5l includedin the circuit 52 which supplies noise currents to the three part channel A.. When currents of any particular range of frequencies are introduced u on the line for the express purpose of pro uc ing noise it. will not be necessary to provide apparatus ieorvselectingv and receiving currents of those particular frequencies'as they represent no message. It is not essential however, 'that a'noise producingdevice that utilizes a range of frequencies n nrner ically equal to that-'cocu ied by a `Joice sg nailing hand used, ut noise currents may he aplied tothe circuit in a narrow band or ands of frequencies. yA special single channel maybe rovided to put these on the line instead ofp using a three part channel as for the conversation A, B and C. For example, the noise currents may he applied to the channel A and the circuits of the 900-1600 and i600-2500 filters may be opened by switches S thereby causing the noise currents to pass through the 500-900 `inittng intelligible signals, for example.

6500-8000 cycles. The frequencies of the llo noise band or bands of frequencies may be above, below or intermingled with the signalling frequencies on the line L depending on the frequency value of the applied noise currents and their frequency fore being applied to the line L.

The apparatus for separating and reassembling the sub-bands arriving at the line terminals 13 will now be described. The subbands A1, B2 and C3 pars directly over the channel 14 to certain appropriate band filters 26 which serve to separate them. The waves corresponding to sub-bands B1, C2. and\A3 traverse a band filter 16 in the channel 15, and are translated successively in -frequency by the demodulators 17 and 18 which are supplied with carrier waves of frequencyY of 6,000 and 8,000 cycles respectively. he band filter 19 which passes a range of frequencies from 8,500 to 10,500 is interposed between the demodulators 17 and 18. The sub-bands corresponding to subbands C1, A2 and B3 are similarly selectedy and translated in frequency successively by band filter 20, demodulator 21, band filter 22 and demodulator 23. Band filters 24 and 25 each pass a range of frequencies of from 500 to 2,500 cycles. Within this range is one sub-band corresponding .to each of the original signal bands which are to be separated out and recombined with the appropriate sub-band which traver:es the circuit 14. It Will be vobserved that the nine band filters 26 serve to perform the final separation and reassembling of the various subbands in the manner necessary to reproduce the original signalling bands corresponding to the speech signals A, B and C, respectively.

In the preceding discussion it has been assumed that the various band filters have an exact and definite cut-ofi` limit. While this is true in theory, it is'not true in practice.. Therefore it must be assumed that each band filter is constructed to transmit freely the band indicated within the required limits of approximation. Thus, for example,

one of the and filters having a transmission limit of from 900 to 1,600 cycles may iny practice be designed to have a transmission range of from 925 to 1,575 cycles, inasmuch as the loss of the small band of frequencies which are thus highly attenuated will not seriously interfere with understanding the conversations transmitted over the system. On the other hand the band filters may be so arranged as to allow a slight overlapping of frequencies, or the original sub-bands may be so chosen and translatedto higher frequencies as to leave a small zone of about 25 to 100 cycles between each sub-band so as to allow for a certain degree of imperfection in the selectivity of the various band filters.

Fig. 3 illustrates a typical band filter cirtranslation becuit such as might be used in any part of the'system of Fig. 2 for the band filters marked BF. The band filter BF of Fig. 3 is one of many types which lnay be employedand consists of any suitable number of sections each having shunt capacity 27 in parallel with shunt inductance 28, and series capacity 29- in series with series inductance 30. The information necessary to enable one skilled in the art to construct a band filter for the purposes herein described is found in Campbell Patent No. 1,227,113,

May 22, 1917.

Fig. 4 is a cie-cuit diagram of a typical form of balanced modulator or demodulator circuit which may be used for translating wave frequencies up or down. The circuits 5, 8, 10,17, 18, 21 and 23 of Fig 2 are such circuit arrangements as shown in Fig. 4 or the equivalentl thereof. The information necessary to enable one skilled in the art to construct and utilize such circuits in the manner and for the purposes herein de-v scribed is found in Carson Patent No. 1,343,307, June 15, 1920, together with Osborne Patents Nos. 1,361,487 and 1,361,488 December 7, 1920.

Fig. 5 is a circuit diagram indicating one manner of applying the invention to a system of duplex or two-way communication. At each terminal of the transmission line L is a three winding transformer or so called hybrid coil 31 which is connected at one end to. the line L and at the other end to a balancing network N whichsimulates the impedance of the line. This enables a transmitting branch 32 and a receiving branch 33 to be connected in conjugate relation with respect to each other at each end of the line. A scrambling or intcrmingling circuit such as shown at the left in Fig. 2 is to be connected to each transmitting branch 32 by connecting the terminals 12 ofthe apparatus at the left of Fig. 2 to the terminals of the branch 32, and an unscra-mbling or separating circuit is to be applied to each receiving branch 33 by connecting the terminals 13 of the apparatus at the right of Fig. 2 to the 'invention to radio transmission. In this case the'circuit to the left of the line terminals 12 is connected at these terminals to the input terminals 34 of a radio transmitting circuit RT.v The radio transmitting circuit as illustrated isof a well known type and includes a modulating vacuum tube or variable impedance device 35, which serves to modulate high frequency waves produced' lilll an article published in the Proceedings of the Institute of Radio Engineers, Volume 9,.

No. 4, August, 1921, page 334. The waves radiated from the antenna 37 are received by the antenna 38 at a distant receiving station and impressed upon a tuned circuit 39 or equivalent selective arrangement such as a band filter, which is connected to the input circuit of any suitable detector. The circuit 39 may be suiliciently broadly tuned to transmit both side bands of the incoming waves representing thev combination of messages, or if, desired, it may be so adjusted 'as to select one side band land the carrier fre- -quency and discriminate against the. other side band. The vacuum tube detector 40 in combination with the receiving antenna 38A and circuit 39, constitute a radio receiver RR of a well known type which needs no further description. To the output circuit Vof the detector 40 is coupled a line 41 which is to be connected to line terminals 13 leading to an unscrambling orseparating circuit such as illustrated at the right of terminals 13 in Fig. 2. The arrangement just described constitutes a one-way radio transmitter, but the invention is equally applicable to two-way or duplex radio transmission. For this "purpose an arrangement such as that shown in Fig. .6a may be used. The line terminals 12 are connected to a modulating device supplied with high frequency waves by the source 56. Modulatcd hgh'frequency waves are impressed upon the -antenna 57 from which they are radiated. Incoming high frequency modulated waves intercepted by the antenna 57 pass through the trap circuit 58 to the detecting device 59 and the resultant detected waves are impressed upon the line terminals 13. The trap circuit 58 comprises an induc- 'tance connected in parallel with a capacity to constitute an anti-resonant circuit which oii'ers high impedance to waves of the transmitting frequency and low impedance to incoming waves to be detected. Both the scrambling and separating circuits will. of

course, be applied at each radio terminal.

Fig. 7 illustrates an arrangement permitting the system of Fig, 2 to be combinedA with an additional modulating arrangement whereby the scrambled band of frequencies maybe used to modulate a carrier wave in a manner similar to well known methods of carrier current transmission. The circuit to the left of line terminals 12, Fig. 2, is connected to input circuit terminals 42 of a vmodulating circuit MC of either of the kinds described in the Proceedings of the Institute of Radio Engineers for August, 1921, on pages 311 and 325. At -the receiving terminal the modulated carrier waves are impressed upon a detector circuit DC of a well known type. To thev output circuit 44 of the detector circuit DC that portion of the circuit of Fig. 2 vto the -right of the line terminals 13 is connected. The circuitarrangement of Fig. 7 is arranged for duplex or two-way` 'communicationin the manner described in connection with Fig. 5. The source 43 which supplies carrier waves to the modulating circuitv MC may, if desired, have a frequency of 6,500 cycles which will produce a lower side band extending from zero to 6,500. The source 43 may also have a frequency of 7,000 cycles which will serve to produce a lower side band of frequencies corresponding in range vtothe original-scrambled band buthaving the several frequencies inverted. The source 43 may also have any `other suitable carrier frequency as, for example, 10,000 cycles or 30,000 cycles.

. The operation of the various systems described .willbe well understood 'from the foregoing description so that no more than a general survey of the operation is necessary. Incoming speech waves from any suitable circuits 1, 2 and 3 lare designated A, B` and' C. Each band .of waves is divi-ded into sub-bands by. means of band filters. A number of these sub-bands A1, B, and C3 are transmitted directly to the outgoing line. The remaining r-sub-bands are translated in frequency by means of modulating and selecting .apparatus so that their frequencies are above those transmitted directly to the line. Waves of frequencies corresponding to all the sub-bands are thus impressed upon the outgoing line, but the various sub-bands are so intermingled with each other that no two sub-bands belonging to the same signaling band lie adjacent each other. At the-receiving terminal a separating circuit serves to separate out sub-bands corresponding to the various sub-bands of the original signaling waves and to translate them back to their original frequencies, whereupon the are assembled to reproduce the origina speech bands, which `are those impressed ,upon receiving circuits corresponding to messages designated A, B and C. 1

The novel features believed to be inherent in the invention are pointed out in the following claims.

IVhat is claimed is: l'

1. The method of multiplex signal transmission from one station to another which comprises dividing each of several bands of frequencies of signal representing waves at one station into sub-bands, translating at least some of the sub-bands to other frequencies and interleaving thesub-bands whereby lull each sub-band except a boundary sub-band is adjacent on each si'de to a sub-band of another band, and transmitting the resultant composite band as a unit to the other station.

2. The method of multiplex speech transmission from one station to another which comprises subdividing each of a plurality of speech Wave bands at one station into subbands, interleaving the sets of sub-bands corresponding to the variousv bands, transmitting the interleaved bands to the other sta'- tion, separating the sub-bands at said other station, and rea-ssembling them to reproduce the original speech waves.

3. In a transmission system, two stations, means at one station for separating each of a plurality of-bands offrequencies of signal representing waves into sub-bands, means for translating at least one sub-band of each` band to a different frequency, means for superimposing on a common circuit all the sub-bands resulting from the translation together with the untranslated bands and means at the other station for separating the subbands, translating the translated sub-bands back to their original values and reassembling Vthe resultant sub-bands to reproduce the original bands.

4. In a speech transmission system, two stations, means at one station for separating several ban-ds of speech frequencies into subbands, means for assembling the sub-bands into groups of sub-bands equal in number to the number of sub-bands into which each band is separated, means for translating certain groups of sub-bands, means for combining the translated and untranslated groupsof sub-bands, means for transmitting them together to the other station, means at the other station for selecting the translated groups of sub-bands and, translating them back to their original values, and means for recombining the translated and untranslated subbands into bands corresponding to the voriginal bands of speech waves.

5. The method of confusing bands of frequencies of waves representing signals which comprises subdividing the bands into subbands so that the sub-bands containing the less important frequencies will have a proportionately larger number of frequencies and superimposing several sub-bands thus produced in such a manner that they cover a continuous rangeof frequencies substantially equal to the sum of the ranges of frequencies of the original bands.

6. The method of subdividing a speech band into subfbands which comprises subdividing the speech band into more than two bands of successively greater frequency range as the frequencies within the band become higher. y

7. The method of confusing signalswhich comprises placing arelatively wide sub-band of frequencies of one signaling band adjacent a relatively narrow sub-band of frequencies of another signaling band.

In witness whereof, I hereunto subscribe my name this 28th day of December A. D.,

RAYMOND A. HEISING. 

